By suggesting the U.S. might not honor its NATO commitment to fight Russia for Estonia, our foreign-policy elites declaimed, Trump has undermined the security architecture that has kept the peace for 65 years. More interesting, however, was the reaction of Middle America. Or, to be more exact, the nonreaction. Americans seem neither shocked nor horrified. What does this suggest? Behind the war guarantees America has issued to scores of nations in Europe, the Mideast and Asia since 1949, the bedrock of public support that existed during the Cold War has crumbled.